The invention relates to bearings for the rotors of electrical motors or magnetic couplings driven by a motor, which have spherical magnetic air gaps between a driven rotor and a driving stator or driving outer magnetic ring. Such spherical motors or spherical magnetic couplings are becoming more and more important in modern pump design. The rotors of spherical motors or couplings are held in position by magnetic forces pressing the rotor against a stationary mounted ball. The impellers of pumps which form a unit with the driven rotor produce hydraulic thrust counteracting the magnetic thrust. When the electricity is switched off, the magnetic force of the driven stator ceases immediately. In contrast, the hydraulic thrust, caused by the difference of pressure between the suction side of the impeller and its backside, generated by the rotation, slowly decreases with decreasing rpm of the impeller. This results in these hydraulic forces tending to lift the impeller with the rotor away from the step bearing after switching the electricity off. The same change in the direction of the axial thrust occurs in magnetic couplings, if the hydraulic forces can exceed the magnetic forces during operation. Therefore it is necessary to support the rotor, not only in the direction of the magnetic forces during constant operation but also to prevent movement in the opposite direction by a ring-shaped, rotating annular member. Spherical bearings consisting of a cap and a ball made from metal are known. Metal balls can be brazed on the tip of a strut. Unfortunately, metal bearings have the tendency to seize when dry friction causes very small fractions of the surface to reach the melting temperatures of the metal causing the cap and the ball to fuse together. Cylindrical bearings produce a film of lubricating liquid which, by hydrodynamic principle, generate high dynamic forces preventing contact between the two members of the bearing. Spherical bearings do not build up such liquid film and therefore, run either dry or semi-dry. Consequently, the probability of seizing is much larger in spherical bearings than in cylindrical bearings. This is the season that metal balls have been replaced by balls made from ceramic materials such as aluminum oxide, circonium oxide or silicon carbide. The melting point of these oxides is so high that it will never be reached by heat of friction; the same applies to silicon carbide, which in addition sublimes and it has no melting point at all.
The disadvantage of ceramic balls is that the ball can be neither welded or brazed onto the strut. It is known to the art pressfit the ball into the hollow strut by pressing the ball into a hole slightly deeper than the radius of the ball, resulting in a useable surface smaller than a hemisphere. These bearings with ceramic balls act only as a step bearing because they can only bear the axial forces in one direction. This restricts their application to spherical rotors in applications where under no circumstances hydraulic forces will not exceed the magnetic forces under any circumstances.